Plasma sputtering a metal or metal nitride with a magnetron having unequal poles

ABSTRACT

A magnetron especially advantageous for low-pressure plasma sputtering or sustained self-sputtering having reduced area but full target coverage. The magnetron includes an outer pole of one magnetic surrounding an inner pole of the other polarity with a gap therebetween. The magnetron is small, primarily located on one side of the central axis, about which it is rotated. The total magnetic flux of the outer pole is at least 1.5 times that of the inner pole. Different shapes include a racetrack, an ellipse, an egg shape, a triangle, and a triangle with an arc conforming to the target periphery. The invention allows increased ionization of the sputtered atoms.

RELATED APPLICATIONS

[0001] This application is a division of Ser. No. 09/928,246, filed Jul.30, 2001, issue fee paid, which is a division of Ser. No. 09/249,468,filed Feb. 12, 1999, now issued as U.S. Pat. No. 6,290,825.

FIELD OF THE INVENTION

[0002] The invention relates generally to sputtering of materials. Inparticular, the invention relates to the magnet and sputteringconditions used to enhance sputtering.

BACKGROUND ART

[0003] Sputtering, alternatively called physical vapor deposition (PVD),is the most prevalent method of depositing layers of metals and relatedmaterials in the fabrication of semiconductor integrated circuits. Aconventional PVD reactor 10 is illustrated schematically in crosssection in FIG. 1, and the illustration is based upon the Endura PVDReactor available from Applied Materials, Inc. of Santa Clara, Calif.The reactor 10 includes a vacuum chamber 12 sealed to a PVD target 14composed of the material, usually a metal, to be sputter deposited on awafer 16 held on a heater pedestal 18. A shield 20 held within thechamber protects the chamber wall 12 from the sputtered material andprovides the anode grounding plane. A selectable DC power supply 22negatively biases the target 14 to about −600 VDC with respect to theshield 20. Conventionally, the pedestal 18 and hence the wafer 16 areleft electrically floating.

[0004] A gas source 24 supplies a sputtering working gas, typically thechemically inactive gas argon, to the chamber 12 through a mass flowcontroller 26. In reactive metallic nitride sputtering, for example, oftitanium nitride, nitrogen is supplied from another gas source 27through its own mass flow controller 26. Oxygen can also be supplied toproduce oxides such as Al₂O₃. The gases can be admitted to the top ofthe chamber, as illustrated, or at its bottom, either with one or moreinlet pipes penetrating the bottom of the shield or through the gapbetween the shield 20 and the pedestal 18. A vacuum system 28 maintainsthe chamber at a low pressure. Although the base pressure can be held toabout 10⁻⁷ Torr or even lower, the pressure of the working gas istypically maintained at between about 1 and 1000 mTorr. A computer-basedcontroller 30 controls the reactor including the DC power supply 22 andthe mass flow controllers 26.

[0005] When the argon is admitted into the chamber, the DC voltagebetween the target 14 and the shield 20 ignites the argon into a plasma,and the positively charged argon ions are attracted to the negativelycharged target 14. The ions strike the target 14 at a substantial energyand cause target atoms or atomic clusters to be sputtered from thetarget 14. Some of the target particles strike the wafer 16 and arethereby deposited on it, thereby forming a film of the target material.In reactive sputtering of a metallic nitride, nitrogen is additionallyadmitted into the chamber 12 and it reacts with the sputtered metallicatoms to form a metallic nitride on the wafer 16.

[0006] To provide efficient sputtering, a magnetron 32 is positioned inback of the target 14. It has opposed magnets 34, 36 creating a magneticfield within the chamber in the neighborhood of the magnets 34, 36. Themagnetic field traps electrons and, for charge neutrality, the iondensity also increases to form a high-density plasma region 38 withinthe chamber adjacent to the magnetron 32. The magnetron 32 is usuallyrotated about the center of the target 14 to achieve full coverage insputtering of the target 14. The form of the magnetron is a subject ofthis patent application, and the illustrated form is intended to be onlysuggestive.

[0007] The advancing level of integration in semiconductor integratedcircuits has placed increasing demands upon sputtering equipment andprocesses. Many of the problems are associated with contact and viaholes. As illustrated in the cross-sectional view of FIG. 2, via orcontact holes 40 are etched through an interlevel dielectric layer 42 toreach a conductive feature 44 in the underlying layer or substrate 46.Sputtering is then used to fill metal into the hole 40 to provideinter-level electrical connections. If the underlying layer 46 is thesemiconductor substrate, the filled hole 40 is called a contact; if theunderlying layer is a lower-level metallization level, the filled hole40 is called a via. For simplicity, we will refer hereafter only tovias. The widths of inter-level vias have decreased to the neighborhoodof 0.25 μm and below while the thickness of the inter-level dielectrichas remained nearly constant at around 0.7 μm. That is, the via holeshave increased aspect ratios of three and greater. For some advancedtechnologies, aspect ratios of six and even greater are required.

[0008] Such high aspect ratios present a problem for sputtering becausemost forms of sputtering are not strongly anisotropic so that theinitially sputtered material preferentially deposits at the top of thehole and may bridge it, thus preventing the filling of the bottom of thehole and creating a void in the via metal.

[0009] It has become known, however, that deep hole filling can befacilitated by causing a significant fraction of the sputtered particlesto be ionized in the plasma between the target 14 and the pedestal 18.The pedestal 18 of FIG. 1, even if it is left electrically floating,develops a DC self-bias, which attracts ionized sputtered particles fromthe plasma across the plasma sheath adjacent to the pedestal 18 and deepinto the hole. Two associated measures of the effective of hole fillingare bottom coverage and side coverage. As illustrated schematically inFIG. 2, the initial phase of sputtering deposits a layer 50, which has asurface or blanket thickness of s₁, a bottom thickness of s₂, and asidewall thickness of s₃. The bottom coverage is equal to s₂/s₁, and thesidewall coverage is equal to S₃/S₁. The model is overly simplified butin many situations is adequate.

[0010] One method of increasing the ionization fraction is to create ahigh-density plasma (HDP), such as by adding an RF coil around the sidesof the chamber 12 of FIG. 1. An HDP reactor not only creates ahigh-density argon plasma but also increases the ionization fraction ofthe sputtered atoms. However, HDP PVD reactors are new and relativelyexpensive. It is desired to continue using the principally DC sputteringof the PVD reactor of FIG. 1.

[0011] Another method for increasing the ionization ratio is to use ahollow-cathode magnetron in which the target has the shape of a top hat.This type of reactor, though, runs very hot and the complexly shapedtargets are very expensive.

[0012] It has been observed that copper sputtered with either aninductively coupled HDP sputter reactor or a hollow-cathode reactortends to form an undulatory film on the via sidewall and the depositedmetal tends to dewet. This is particularly serious when the sputteredcopper layer is being used as a seed layer for a subsequent depositionprocess such as electroplating to complete the copper hole filling.

[0013] A further problem in the prior art is that the sidewall coveragetends to be asymmetric with the side facing the center of the targetbeing more heavily coated than the more shielded side. Not only doesthis require excessive deposition to achieve a seed layer ofpredetermined thickness, it causes cross-shaped trenches used asalignment indicia in the photolithography to appear to move as thetrenches are asymmetrically narrowed.

[0014] Another operational control that promotes deep hole filling islow chamber pressure. At higher pressures, there is a higher probabilitythat sputtered particles, whether neutral or ionized, will collide withatoms of the argon carrier gas. Collisions tend to neutralize ions andto randomize velocities, both effects degrading hole filling. However,as described before, the sputtering relies upon the existence of aplasma at least adjacent to the target. If the pressure is reduced toomuch, the plasma collapses, although the minimum pressure is dependentupon several factors.

[0015] The extreme of low-pressure plasma sputtering is sustainedself-sputtering (SSS), as disclosed by Fu et al. in U.S. patentapplication Ser. No. 08/854,008, filed May 8, 1997. In SSS, the densityof positively ionized sputtered atoms is so high that a sufficientnumber are attracted back to the negatively biased target to resputtermore ionized atoms. No argon working gas is required in SSS. Copper isthe metal most prone to SSS, but only under conditions of high power andhigh magnetic field. Copper sputtering is being seriously developedbecause of copper's low resistivity and low susceptibility toelectromigration. However, for copper SSS to become commerciallyfeasible, a full-coverage, high-field magnetron needs to be developed.

[0016] Increased power applied to the target allows reduced pressure,perhaps to the point of sustained self-sputtering. The increased poweralso increases the ionization density. However, excessive power requiresexpensive power supplies and increased cooling. Power levels in excessof 20 to 30 kW are considered infeasible. In fact, the pertinent factoris the power density in the area below the magnetron since that is thearea of the high-density plasma promoting effective sputtering. Hence, asmall, high-field magnet would most easily produce a high ionizationdensity. For this reason, apparently, some prior art discloses a smallcircularly shaped magnet. However, such a magnetron requires not onlyrotation about the center of the target to provide uniformity, but italso requires radial scanning to assure full and fairly uniform coverageof the target. If full magnetron coverage is not achieved, not only isthe target not efficiently used, but more importantly the uniformity ofsputter deposition is degraded, and some of the sputtered materialredeposits on the target in areas that are not being sputtered. Theredeposited material builds up to such a thickness that it is prone toflake off, producing severe particle problems. While radial scanning canpotentially avoid these problems, the mechanisms are complex andgenerally considered infeasible.

[0017] One type of commercially available magnetron is kidney-shaped, asexemplified by Tepman in U.S. Pat. No. 5,320,728. Parker discloses moreexaggerated forms of this shape in U.S. Pat. No. 5,242,566. Asillustrated in plan view in FIG. 3, the Tepman magnetron 52 is based ona kidney shape for the magnetically opposed pole faces 54, 56 separatedby a circuitous gap 57 of nearly constant width. The pole faces 54, 56are magnetically coupled by unillustrated horseshoe magnets. Themagnetron rotates about a rotational axis 58 at the center of the target14 and near the concave edge of the kidney-shaped inner pole face 54.The curved outer periphery of the outer pole face 56, which is generallyparallel to the gap 57 in that area, is close to the outer periphery ofthe usable portion if the target 14. This shape has been optimized forhigh field and for uniform sputtering but has an area that is nearlyhalf that of the target. It is noted that the magnetic field isrelatively weak in areas separated from the pole gap 57.

[0018] For these reasons, it is desirable to develop a small, high-fieldmagnetron providing full coverage so as to promote deep hole filling andsustained copper self-sputtering.

SUMMARY OF THE INVENTION

[0019] The invention includes a sputtering magnetron having an oval orrelated shape of smaller area than a circle of equal diameter where thetwo diameters extend along the target radius with respect to the typicalrotation axis of the magnetron. The shapes include racetracks, ellipses,egg shapes, triangles, and arced triangles. The magnetron is rotated onthe backside of the target about a point preferably near the magnetron'sthin end and the thicker end is positioned more closely to the targetperiphery.

[0020] The invention also includes sputtering methods achievable withsuch a magnetron. Many metals not subject to sustained self-sputteringcan be sputtered at chamber pressures of less than 0.5 milliTorr, oftenless than 0.2 milliTorr, and even at 0.1 milliTorr. The bottom coveragecan be further improved by applying an RF bias of less than 250W to apedestal electrode sized to support a 200 mm wafer. Copper can besputtered with 18 kW of DC power for a 330 mm target and 200 mm wafereither in a fully self-sustained mode or with a minimal chamber pressureof 0.3 milliTorr or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic diagram of a DC plasma sputtering reactor.

[0022]FIG. 2 is a cross-sectional view of a inter-level via in asemiconductor integrated circuit.

[0023]FIG. 3 is a plan view of a conventional magnetron.

[0024]FIG. 4 is a plan view of the pole pieces of an embodiment of themagnetron of the invention taken along the view line 4-4 of FIG. 6.

[0025]FIG. 5 is a plan view of the magnets used in the magnetron of FIG.4.

[0026]FIG. 6 is a cross-sectional view of one of the magnets used inconjunction with the embodiments of the invention.

[0027]FIG. 7 is a cross-sectional view of the magnetron of FIG. 4.

[0028]FIG. 8 is a plan view of an egg-shaped magnetron.

[0029]FIG. 9 is a plan view of a triangularly shaped magnetron.

[0030]FIG. 10 is a plan view of a modification of the triangularlyshaped magnetron of FIG. 9, referred to as an arced triangularmagnetron.

[0031]FIG. 11 is a plan view of the magnets used in the arced triangularmagnetron of FIG. 10.

[0032]FIG. 12 is a plan view of two model magnetrons used to calculateareas and peripheral lengths.

[0033]FIG. 13 is a graph of the angular dependences of the areas of atriangular and of a circular magnetron.

[0034]FIG. 14 is a graph of the angular dependences of the peripherallengths of the two types of magnetrons of FIG. 12.

[0035]FIG. 15 is a side view of an idealization of the magnetic fieldproduced with the described embodiments of the invention.

[0036]FIG. 16 is a graph showing the effect of RF wafer bias in bottomcoverage in titanium sputtering.

[0037]FIG. 17 is a graph of the dependence of chamber pressure uponnitrogen flow illustrating the two modes of deposition obtained inreactive sputtering of titanium nitride with a magnetron of theinvention.

[0038]FIG. 18 is a graph of the step coverage obtained in the twosputtering modes for reactive sputtering of titanium nitride with amagnetron of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0039] One embodiment of the invention is a racetrack magnetron 60,illustrated in plan view in FIG. 4, having a central bar-shaped poleface 62 of one magnetic polarity having opposed parallel middle straightsides 64 connected by two rounded ends 66. The central pole face 62 issurrounded by an outer elongated ring-shaped pole face 68 of the otherpolarity with a gap 70 between them. The outer pole face 68 of the othermagnetic polarity includes opposed parallel middle straight sections 72connected by two rounded ends 74 in general central symmetry with theinner pole face 62. The middle sections 72 and rounded ends 74 are bandshaving nearly equal widths. Magnets, to be described shortly, cause thepole faces 62, 68 to have opposed magnetic polarities. A backing plate,also to be described shortly, provides both a magnetic yoke between thepole faces 62, 68 and support for the magnetron structure.

[0040] Although the two pole faces 62, 68 are illustrated with specificmagnetic polarities producing magnetic fields extending generallyperpendicularly to the plane of illustration, it is of courseappreciated that the opposite set of magnetic polarities will producethe same general magnetic effects as far as the invention is concerned.This assembly produces a generally semi-toroidal field extendingperpendicularly to a closed path with a minimal field-free region in thecenter. It is intended that the pole assembly of FIG. 4 be rotated abouta rotation axis 78 approximately coincident with the center of thetarget 14 and located at or near one prolate end 80 of the outer poleface 68 and with its other prolate end 82 located approximately at theouter radial usable extent of the target 14, thereby achieving fulltarget coverage. The outer usable periphery of the target is not easilydefined because different magnetron designs use different portions ofthe same target. However, it is bounded by the flat area of the targetand almost always extends to significantly beyond the diameter of thewafer being sputter deposited and is somewhat less than the area of thetarget face. For 200 mm wafers, target faces of 325 mm are typical. A15% unused target radius may be considered as an upper practical limit.

[0041] As illustrated in the plan view of FIG. 5, two sets of magnets90, 92 are disposed in back of the pole faces 62, 68 to produce the twomagnetic polarities. The magnets 90, 92 are of similar construction andcomposition producing an axially extending magnetic flux on eachvertically facing end. A cross-sectional view of a magnet 90, 92 isshown in FIG. 6. A cylindrical magnetic core 93 extending along an axisis composed of a strongly magnetic material, such as neodymium boroniron (NdBFe). Because such a material is easily oxidized, the core 93 isencapsulated in a case made of a tubular sidewall 94 and two generallycircular caps 96 welded together to form an air-tight canister. The caps96 are composed of a soft magnetic material, preferably SS410 stainlesssteel, and the tubular sidewall 96 is composed is composed of anon-magnetic material, preferably SS304 stainless steel. Each cap 96includes an axially extending pin 97, which engages a correspondingcapture hole in one of the pole faces 62, 68 or in a magnetic yoke to beshortly described. Thereby, the magnets 90, 92 are fixed in themagnetron. The magnetic core 93 is magnetized along its axial direction,but the two different magnets 90, 92 are oriented in the magnetron 60,as illustrated in the cross-sectional view of FIG. 7, so that themagnets 90 of the inner pole 62 are aligned to have their magnetic fieldextending vertically in one direction, and the magnets 92 of the outerpole 68 are aligned to have their magnetic field extending vertically inthe other direction. That is, they have opposed magnetic polarities.

[0042] As illustrated in the cross-sectional view of FIG. 6, the magnets90, 92 are arranged closely above (using the orientation of FIG. 1) thepole faces 62, 68 located just above the back of the target 14. Amagnetic yoke 98 having a generally closed shape generally conforming tothe outer periphery of the outer pole face 68 is closely positioned inback of the magnets 90, 92 to magnetically coupled the two poles 62, 68.As mentioned previously, holes in the pole faces 68, 72 and in the yoke98 fix the magnets 90, 92.

[0043] The inner magnets 90 and inner pole face 62 constitute an innerpole of one magnetic polarity while the outer magnets 92 and the outerpole face 72 constitute a surrounding outer pole of the other magneticpolarity. The magnetic yoke 98 magnetically couples the inner and outerpoles and substantially confines the magnetic field on the back or topside of the magnetron to the yoke 98. A semi-toroidal magnetic field 100is thereby produced, which extends through the non-magnetic target 14into the vacuum chamber12 to define the high-density plasma region 38.The field 100 extends through the non-magnetic target 14 into the vacuumchamber 12 to define the extent of the high-density plasma region 38. Asillustrated, the magnetron 60 extends horizontally from approximatelythe center of the target 14 to the edge of the usable area of the target14. The magnetic yoke 90 and the two pole faces 62, 68 are preferablyplates formed of a soft magnetic material such as SS416 stainless steel.

[0044] The inner prolate end 80 of the magnetron 60 is connected to ashaft 104 extending along the rotation axis 78 and rotated by a motor106. As illustrated, the magnetron 60 extends horizontally fromapproximately the center of the target 14 to the right hand side of theusable area of the target 14. Demaray et al. in U.S. Pat. No. 2,252,194disclose exemplary details of the connections between the motor 106, themagnetron 60, and the vacuum chamber 12. The magnetron assembly 60should include counter-weighting to avoid flexing of the shaft 104.Although the center of rotation 78 is preferably disposed within theinner prolate end 74 of the outer pole face 72, its position may beoptimized to a slightly different position, but one preferably notdeviating more than 20% from the inner prolate end 80 as normalized tothe prolate length of the magnetron 60.

[0045] The racetrack configuration of FIG. 4 can be alternativelycharacterized as an extremely flattened oval. Other oval shapes are alsoincluded within the invention, for example, elliptical shapes with themajor axis of the ellipse extending along the radius of the target andwith the minor axis preferably parallel to a rotational circumference.

[0046] Another oval shape is represented by an egg-shaped magnetron 106,illustrated in plan view in FIG. 8, having an outer pole face 108 of onemagnetic polarity surrounding an inner pole face 110 of the otherpolarity with a gap 122 between them. Both pole faces 108, 110 areshaped like the outline of an egg with a major axis extending along theradius of the target. However, an inner end 112 of the outer pole face108 near the rotation axis 78 is sharper than an outer end 114 near theperiphery of the target. The egg shape is approximately elliptical butis asymmetric with respect to the target radius. Specifically, the minoraxis is pushed closed to the target periphery. The inner pole face 110and the gap 122 are similarly shaped.

[0047] A related shape is represented by a triangular magnetron 126,illustrated in plan view in FIG. 9, having a triangular outer pole face128 of one magnetic polarity surrounding a substantially solid innerpole face 130 of the other magnetic polarity with a gap 132. Thetriangular shape of the inner pole face 130 with rounded corners allowshexagonal close packing of the button magnets. The outer pole face 128has three straight sections 134 are preferably offset by 60° withrespect to each other and are connected by rounded corners 136.Preferably, the rounded corners 136 have smaller lengths than thestraight sections 134.

[0048] A modified triangular shape is represented by an arced triangularmagnetron 140 of FIG. 10. It includes the triangular inner pole face 130surrounded by an arced triangular outer pole face 142 with a gap 144between them and between the magnets of the respective poles and withthe magnetic yoke in back of the gap 144. The outer pole face 142includes two straight sections 146 connected to each other by a roundedapex corner 148 and connected to an arc section 150 by roundedcircumferential corners 152. The rotational center 78 is located nearthe apex corner 148. The arc section 150 is located generally near thecircumferential periphery of the target. It curvature may be equal tothat of the target, that is, be equidistant from the center of rotation78, but other optimized curvatures may be chosen for an arc sectionconcave with respect to the rotational center 78.

[0049] The magnetic field is produced by an arrangement of magnets shownin plan view in FIG. 11. Magnets 160 of a first polarity are disposed ina hexagonally close-packed arrangement adjacent to the inner pole face130. Magnets 162 of a second polarity are arranged adjacent to the arcsection 150 of the outer pole face 142 while magnets 164 of the secondpolarity are arranged adjacent to the remaining portions of the outerpole face 142. In some situations, to be described later, it isadvantageous to place magnets of different intensities at differentportions of the outer pole face 142. In one embodiment, there are 10magnets in the inner pole and 26 magnets in the outer pole, which formagnets of equal strength produces 2.6 more magnetic flux in the outerpole than in the inner pole.

[0050] The triangular magnetrons 126, 140 of FIGS. 9 and 10 areillustrated as having apex angles θ of 60°, but the apex angle can bechanged, in particular decreased below 60°, although 60°±15° seems toprovide superior uniformity. The apex angle significantly affects twoimportant parameters of the magnetron of the invention, the values ofits area A and its perimeter P. Some simple calculations, most easilydone for the arced triangular magnetron 140, show the general effects ofchanging the apex angle θ, as illustrated in plan view in FIG. 12. Asimplified arced triangular magnetron 170 has two straight sidesextending between the center and periphery of the target 14 of radius RTand meeting at an apex coincident with the rotation axis 78 and furtherincludes an arc side conforming to the usable periphery of the target14. The area A of the simplified arced triangular magnetron 170 isθR_(T) ², and its periphery P is R_(T)(2+θ), where θ is measured inradians. Also illustrated in FIG. 12 is a circular magnetron 172 havinga radius of R_(T)/2 and having a diameter fixed to the rotation axis 78.It has an area A of πR_(T) ²/4 and a periphery P of ART. Both magnetrons170, 172 provide full target coverage. The dependence of the arcedtriangular area A upon the apex angle θ is plotted in normalized unitsin FIG. 13 by line 174 and that for the circular area by line 176. Below45°, the triangular area is smaller. The dependence of the triangularperiphery P is plotted in FIG. 14 by line 178 and that for the circularperiphery by line 180. Below 65.4°, the arced triangular periphery issmaller. Ionization efficiency is increased by minimizing the area,since the target power is concentrated in a smaller area, and is alsoincreased by minimizing the periphery, since edge loss is generallyproportional to the peripheral length. Of course, the area needs to belarge enough to accommodate the magnets creating the magnetic field.Also, these calculations do not address uniformity.

[0051] The shapes presented above have all been symmetric about thetarget radius. However, the magnetron of the invention includesasymmetric shapes, for example one radially extending side being in theform of the racetrack of FIG. 4 and the other side being oval, e.g., theegg shape of FIG. 7, or one radially extending side being oval orstraight and the other side having a triangular apex between the centerand periphery of the target.

[0052] All the magnetrons described above have asymmetric inner andouter poles. In particular, the total magnetic flux ∫B·dS produced bythe inner pole 190, illustrated schematically in FIG. 15, is much lessthan that produced by the surrounding outer pole 192, for example, by atleast a factor of 1.5 and preferably 2. All the magnetrons are alsocharacterized as having a compact inner pole 190 surrounded by the outerpole 192. The result is a magnetic field distribution which is verystrong in the reactor processing area 194 adjacent to the gap 196between the poles 190, 192, but which also extends far into theprocessing area 194 as the magnetic field lines of the outer pole 192close back to the magnetic yoke 198. The substantial fraction ofmagnetic field extending vertically from the target deep into theprocessing area 194 helps to support a sustained self-sputtering plasmadeep into the processing area 194 and to guide ionized sputteredparticles toward the wafer.

[0053] The inventive magnet achieves a relatively high magnetic field.However, magnetic field intensity of itself is insufficient. Someconventional magnetrons, such as Demaray et al. disclose in theaforecited patent, use a line of horseshoe magnets arranged in akidney-shaped linear path with only a small gap between the poles of thehorseshoes. As a result, a relatively high magnetic field intensity canbe achieved in the area at the periphery of the kidney shape. However,the linear shape of the high magnetic field surrounds an area ofsubstantially no magnetic field. As a result, electrons can escape tonot only the exterior but also the interior of the high-field region. Incontrast, the inner pole of the triangular magnetron of the inventionproduces a magnetic cusp of minimal area. If electrons are lost from themagnetic field on one side of the inner pole, they are likely to becaptured on the other side, thus increasing the plasma density for agiven power level. Furthermore, the inner pole includes a singlemagnetizable pole face producing a generally uniform magnetic flux. Ifmultiple inner poles faces were used for multiple inner magnets,magnetic field lines would extend to between the inner magnets.

[0054] A further advantage of the inventive design is that one pole isformed in a closed line and surrounds the other pole. It would bepossible to form a very small linearly extending magnetron with highmagnetic field intensity by arranging horseshoe magnets or the like inan open ended line with the two sets of poles being closely spaced.However, the electrons could then easily escape from the open ends anddecrease the density of the plasma.

[0055] It is understood that the shapes described above refer to polefaces having band-like widths of area not significantly larger than thebutton magnets being used. The widths, particularly of the outer poleface, can be increased, perhaps even non-uniformly, but the additionalwidth is of less effectiveness in generating the desired high magneticfield.

[0056] It is believed that the beneficial results of the invention areachieved in large part because the oval magnetrons and magnetrons ofrelated shapes produce a higher plasma ionization density withoutrequiring excessive power. Nonetheless, full target coverage isachieved. In one aspect, the inventive magnetron has a relatively smallarea, but has a shape that allows full target coverage without radialscanning. The triangular magnetron 160 of FIG. 10 with an apex angle of60° has an area of ⅙ (0.166) of the usable target area. In contrast, ifthe circular magnetron 162 were used, which similarly extends from thetarget center to the periphery, the magnetron area is ¼ (0.25) of thetarget area. As a result, the power density is less for a given powersupply powering a larger circular magnetron. The target overlaypercentage is even higher for the Tepman magnet of FIG. 3.

[0057] The combination of small area and full coverage is achieved by anouter magnetron shape extending from the target center to its usableperiphery (±15%) and having a transverse dimension at half the targetradius of less substantially less than the target radius, that is,prolate along the target radius. The transverse dimension should bemeasured circumferentially along the rotation path.

[0058] The uniformity is enhanced by an oval shape that is transverselywider, with a respect to the target radius, at its outer end near thetarget periphery than at its inner end near the center of rotation. Thatis, the minor axis is displaced towards the target circumference.

Processes

[0059] An arced triangular magnetron of the invention was tested inseveral experiments. For almost all the experiments, the target wasspaced between 190 and 200 mm from the wafer and the target had adiameter of 330 mm for a 200 mm wafer.

[0060] For copper sputtering, uniformity is improved by using ten strongmagnets 160 in the inner pole, strong magnets 162 along the arc portion150 of the outer pole, and weaker magnets 164 for the remainder of theouter pole. The stronger magnets have a diameter 30% larger than thediameter of the weaker magnets, but are otherwise of similar compositionand structure, thereby creating an integrated magnetic flux that is 70%larger.

[0061] Sustained self-sputtering of copper is achieved, after strikingthe plasma in an argon ambient, with 9 kW of DC power applied to thetarget having a usable diameter of about 30 cm. However, it isconsidered desirable to operate with 18 kW of DC power and with aminimal argon pressure of about 0.1 milliTorr arising at least in partfrom leakage of the gas providing backside cooling of the wafer to theliquid-chilled pedestal. The increased background pressure of 0.1 to 0.3milliTorr enhances effective wafer cooling without significant increasein the scattering and deionization of the sputtered ions. Theserelatively low DC powers are important in view of the ongoingdevelopment of equipment for 300 mm wafers, for which these numbersscale to 20 kW and 40 kW. A power supply of greater than 40 kW isconsidered expensive if not infeasible.

[0062] One application of ionized copper sputtering is to deposit a thinconformal seed layer in a deep and narrow via hole. Thereafter, electroor electroless plating can be used to quickly and economically fill theremainder of the hole with copper.

[0063] In one experiment, a via hole having a top width of 0.30 μm andextending through 1.21 μm of silica was first coated with a Ta/TaNbarrier layer. Copper was deposited over the barrier layer at 18 kW oftarget power and a pressure of 0.2 milliTorr to a blanket thickness ofabout 0.18 μm. The sides of the via hole was smoothly covered. Thesidewall thickness of the copper was about 7 nm on one side and 11.4 nmon the other side for a via located at the wafer edge. The bottomthickness was about 24 nm. Sidewall symmetry was improved for a via holeat the wafer center. The smoothness promotes the use of the depositedlayer as a seed layer and as an electrode for subsequent electroplatingof copper. The relatively good symmetry between the two sidewallsrelieves the problem in the prior art of moving photolithographicindicia.

[0064] Sputtering of an aluminum target was achieved at both 12 kW and18 kW of applied power with a minimum pressure of about 0.1 milliTorr, asignificant improvement. For aluminum sputtering, sidewall coverage andparticularly bottom coverage were significantly improved. The betteruniformity is also believed to be related in part to the increasedionization fraction since the self-biased pedestal supporting the waferattracts the ionized sputtered particles across its entire area. It isestimated that the magnetron of the invention increases the ionizationfraction from 2% to at least 20% and probably 25%.

[0065] The arced triangular magnetron was compared under similaroperating conditions to the operation of a conventional magnetronresembling the Tepman magnetron of FIG. 3. The comparative results aresummarized in TABLE 1 for the sputtering of aluminum. TABLE 1 TriangleConv. Bottom 28.5% 8.0% Coverage Sidewall 8.0% 5.7% Coverage Uniformity4.6% 7.8% (190 mm) Uniformity 3.0% 6.0% (290 mm) Minimum 0.1 0.35Pressure (milliTorr)

[0066] The coverage results were obtained for via holes having a widthof 0.25 μm and a depth of 1.2 μm, that is, an aspect ratio of about 5.The bottom coverage is significantly improved with the inventivetriangular magnetron compared to the conventional magnetron. Thesidewall coverage is also increased, and further the coverage is smoothand uniform from top to bottom. These two characteristics promote theuse of the deposited metal layer as a seed layer for a subsequentdeposition step. This is particularly important for copper in which thesecond deposition is performed by a different process such aselectroplating. The increased bottom and sidewall coverages are believedto be due to the higher ionization fraction of sputtered aluminum atomsachieved with the inventive triangular magnetron. This ionizationfraction is believed to be 25% or greater. The uniformity of blanket(planar) deposition was determined both for a separation of 190 mmbetween the target and the wafer and, in a long-throw implementation,for a separation of 290 mm. The inventive triangular magnetron producesbetter uniformity, especially for long throw. The better uniformity isalso believed to be related to the increased ionization fraction sincethe self-biased pedestal supporting the wafer attracts the ionizedsputtered particles across its entire area. Similarly, the inventivetriangular magnetron produces less asymmetry between the coverages ofthe two opposed sidewalls. The increased ionization density is due inpart to the relatively small inner yoke having an area substantiallyless than that of the outer yoke. As a result, electrons lost from oneside of the inner yoke are likely to be captured by the other side.

[0067] The arced triangular magnetron was also used to sputter titanium.Titanium, sometimes in conjunction with titanium nitride, is useful inaluminum metallization for providing a silicided contact to silicon atthe bottom of a contact hole and to act as barrier both to the siliconin a contact hole and between the aluminum and the silica dielectric onthe via or contact sidewalls. Conformal and relatively thick coatingsare thus required.

[0068] A series of experiments were performed using a titanium targetwith 18 kW of DC target power and with only six magnets 160 in the innerpole. At a chamber pressure of 0.35 milliTorr, the bottom coverage anduniformity are observed to be good.

[0069] The titanium experiments were continued to measure bottomcoverage as a function of the aspect ratio (AR) of the via hole beingcoated. With no wafer bias applied and the pedestal heater 18 leftelectrically floating, the 18 kW of target power nonetheless self-biasesthe target to about 30 to 45V. The bottom coverage under theseconditions is shown by line 190 in the graph of FIG. 15. The bottomcoverage decreases for holes of higher aspect ratios but is still anacceptable 20% at AR=6.

[0070] In a continuation of these experiments, an RF power source 192,illustrated in FIG. 1, was connected to the heater pedestal 18 through acoupling capacitor circuit 194. It is known that such an RF fieldapplied to the wafer adjacent to a plasma creates a DC self-bias. When100W of 400 kHz power is applied, the bottom coverage is significantlyincreased, as shown by line 196 in the graph of FIG. 16. These powersshould be normalized to a 200 mm circular wafer. However, when the biaspower is increased to 250W, resputtering and faceting of the top cornersof the via hole becomes a problem. The bottom coverage results for 250Wbias are shown by line 198. They are generally worse than for 100 W ofwafer bias. A higher bias frequency of 13.56 MHz should provide similarresults.

[0071] The magnetron of the invention can also be used for reactivesputtering, such as for TiN, in which nitrogen is additionally admittedinto the chamber to react with the sputtered metal, for example, withtitanium to produce TiN or with tantalum to produce TaN. Reactivesputtering presents a more complex and varied chemistry. Reactivesputtering to produce TiN is known to operate in two modes, metallicmode and poison mode. Metallic mode produces a high-density,gold-colored film on the wafer. Poison mode, which is often associatedwith a high nitrogen flow, produces a purple/brown film whichadvantageously has low stress. However, the poison-mode film has manygrain boundaries, and film defects severely reduce chip yield.Furthermore, the deposition rate in poison mode is typically onlyone-quarter of the rate in metallic mode. It is generally believed thatin poison mode the nitrogen reacts with the target to form a TiN surfaceon the Ti target while in metallic mode the target surface remains cleanand TiN forms only on the wafer.

[0072] The arced triangular magnetron was used for reactive sputteringof titanium nitride. The initialization conditions are found to be veryimportant to obtain operation in metallic mode. In a series of initialexperiments, argon alone is first admitted to the chamber. After theplasma is struck at an argon pressure of about 0.5 milliTorr, the argonflow is reduced to 5 sccm producing a pressure of 0.3 milliTorr. Whenthe nitrogen flow is then step wise ramped up to 100 sccm and then isgradually reduced, the dependence of the chamber pressure upon the flowassumes a hysteretic form illustrated in FIG. 17. Between about 50 and70 sccm of nitrogen, intermediate ramp-up pressures 200 are belowcorresponding intermediate ramp-down pressures 202. At lower pressures204 and at higher pressures 206, there is no significant separationbetween ramp up and ramp down. It is believed that the lower pressures204 and intermediate ramp-up pressures 200 cause sputtering in metallicmode while higher pressures 206 and intermediate ramp-down pressures 202cause sputtering in poison mode.

[0073] These results show that, for higher operational deposition ratesin the generally preferred metallic mode, it is important to not exceedthe intermediate ramp-up pressures 200, that is, not to exceed themaximum metallic-mode flow, which in these experiments is 70 sccm orslightly higher but definitely below 80 sccm. The argon and nitrogen canbe simultaneously and quickly turned on, but preferably the DC power isalso quickly turned on.

[0074] There are some applications, however, where operation in poisonmode is preferred. This can be achieved by first going to the higherpressures 206 and then decreasing to the ramp-down intermediatepressures 202. Alternatively, poison mode can be achieved by immediatelyturning on the desired gas flow, but only gradually turning on the DCsputtering power supply at a rate of no more than 5 kW/s.

[0075] Titanium nitride was sputtered into high aspect-ratio via holesin both metallic and poison modes at a N₂ flow of 50 sccm and an Ar flowof 5 sccm after the plasma had been struck in argon. These flows producea pressure of 1.7 milliTorr in metallic mode and 2.1 milliTorr in poisonmode. The deposition rates are 100 nm/min in metallic mode and 30 nm/minin poison mode. On one hand, the TiN film stress is higher when it isdeposited in metallic mode, but on the other hand poison mode suffersfrom overhang and undulatory sidewall thicknesses near the top of thevia hole. A series of experiments deposited TiN into via holes ofdiffering aspect ratios. The resulting measured bottom coverage,illustrated in the graph of FIG. 18, shows in line 210 that bottomcoverage in metallic mode remains relatively high even with an viaaspect ratio of 5 while in line 212 the step coverage in poison mode isalways lower and drops dramatically for aspect ratios of four andhigher.

[0076] The inventive magnetron can also be used to sputter deposit othermaterials, for example, W using a tungsten target, or TaN, using atantalum target and nitrogen gas in the plasma. Reactive sputtering ofWN is also contemplated.

[0077] The magnetron of the invention is thus efficient in producing ahigh ionization fraction because of the high-density plasma it cancreate without excessive power being required. Nonetheless, its fullcoverage allows for uniform deposition and full target utilization. Itssputtering uniformity is good. Nonetheless, no complex mechanisms arerequired.

[0078] Such a small, high-field magnet enables sustained self-sputteringwith relatively modest target power and also enables sputtering ofmaterials such as aluminum and titanium at reduced pressures below 0.5milliTorr, preferably below 0.2 milliTorr, and even at 0.1 milliTorr. Atthese pressures, deep hole filling can be facilitated by the reducedscattering of sputtered particles, whether neutral or ionized, and bythe reduced neutralization of ionized particles. The high-field magnetfurther promotes a high ionization fraction, which can be drawn into adeep, narrow hole by biasing of the wafer within proper ranges.

1. A method of sputtering material from a target comprising a metal ontoa working substrate supported on a pedestal in a system including amagnetron disposed on a side of said target opposite said pedestal alonga central axis of a vacuum chamber containing said pedestal andincluding an outer pole having a first magnetic polarity and a firsttotal magnetic flux and an inner pole surrounded by said outer pole andhaving a second magnetic polarity opposite said first magnetic polarityand a second total magnetic flux which is larger than said first totalmagnetic flux by a factor of at least 1.5, said method comprising thesteps of: rotating said magnetron about said central axis; admitting aworking gas into said vacuum chamber; and applying DC power to saidtarget to excite said working gas into a plasma to thereby sputter saidmetal of said target onto said substrate.
 2. The method of claim 1,wherein said metal is tantalum.
 3. The method of claim 1, wherein saidmetal is titanium.
 4. The method of claim 1, wherein said metal istungsten.
 5. The method of claim 1, further comprising admitting gaseousnitrogen into said vacuum chamber, wherein a nitride of said metal isformed on said substrate.
 6. The method of claim 5, wherein said metalis tantalum.
 7. The method of claim 5, wherein said metal is titanium.8. The method of claim 5, wherein said metal is tungsten.
 9. The methodof claim 1, wherein said factor is at least 2.0.
 10. The method of claim1, wherein an area within a periphery of said magnetron is no more than⅙ of a usable area of said target.
 11. The method of claim 1, furthercomprising RF biasing said pedestal.
 12. A tantalum sputtering methodperformed in a plasma sputter reactor having a tantalum target disposedon one side of a vacuum chamber and arranged about a central axis,comprising the steps of: supporting a substrate to be sputter coated ona pedestal electrode arranged opposite said target along said centralaxis; rotating a magnetron disposed on a side of said target oppositesaid pedestal about said central axis, said magnetron including an innerpole of a first magnetic polarity and having a first total magnetic fluxand an outer pole of a second magnetic polarity opposite said firstmagnetic polarity, having a second total magnetic flux greater than saidfirst total magnetic flux by a factor of at least 1.5, and surroundingsaid first magnetic pole; admitting argon into said vacuum chamber;applying negative DC power to said target to excite said argon into aplasma to sputter said target; and RF biasing said pedestal electrode toinduce a negative DC self-bias thereupon.
 13. The method of claim 12,wherein said factor is at least 2.0.
 14. The method of claim 12, whereinan area within a periphery of said magnetron is no more than ⅙ of anarea of said target.
 15. The method of claim 12, further comprisingadmitting nitrogen into said vacuum chamber, whereby tantalum nitride isdeposited on said substrate.
 16. A tantalum plasma sputter reactor,comprising: a vacuum chamber; a tantalum target disposed on a side ofsaid vacuum chamber; a pedestal electrode disposed in said vacuumchamber in opposition to said target for supporting a substrate to besputter coated; and a magnetron rotatable about said central axis andincluding an inner magnetic pole having a first magnetic polarity and afirst total magnetic flux and an outer magnetic pole surrounding saidinner magnetic pole and having a second magnetic polarity opposite saidfirst magnetic polarity and a second total magnetic flux greater thansaid first total magnetic flux by a ratio of at least 1.5.
 17. Thereactor of claim 16, wherein said ratio is at least 2.0.
 18. The reactorof claim 16, wherein an area within a periphery of said magnetron is nomore than ⅙ of an area of said target.
 19. The reactor of claim 16,further comprising an RF power supply connected to said pedestalelectrode.